1. Field of the Invention
This invention generally relates to shutters for optical apparatuses, and more particularly, to a shutter suitable for use in an optical apparatus such as a digital camera or camcorder.
2. Description of the Related Art
With respect to a shutter of a camera, there is a description in, for example, Japanese Patent Application Publication No. 2000-39646 (hereinafter, referred to as Document 1). Document 1 discloses a technique of driving opening and closing of two sectors, which are an upper aperture blade and a lower aperture blade, by use of a stepping motor. FIG. 14 shows the conventional sector operation described in Document 1. A first sector 101 is pivotally provided with respect to a fixed shaft 102, and a second sector 107 is pivotally provided with respect to a fixed shaft 108. These sectors 101 and 107 are movably arranged in positions that open and close a shutter opening 100 provided in a shutter substrate.
Specifically, an opening 103 is provided in the first sector 101. In the same manner, an opening 109 is provided in the second sector 107. The openings 103 and 109 are pierced by an operation pin 105, and are engaged with each other, the operation pin 105 being swung by the stepping motor, not shown. Thus, as shown in FIG. 14, it is possible to form a small aperture state, fully closed state, and fully open state by moving the operation pin 105 in a given direction to control the positions of the two sectors 101 and 107.
In the shutter disclosed in Document 1, the openings 103 and 109 provided in the first sector 101 and the second sector 107 have simple structures. As the operation pin 105 moves upwardly, the above-mentioned first sector 101 and the second sector 107 form the small aperture state, fully closed state, and fully open state. In contrast, if the operation pin 105 moves downwardly from the fully open state, an opposite state is formed. Here, the fully closed state is existent between the small aperture state and the fully open state.
Now, an inertial force (inertia) works, when the sectors 101 and 107 are moved from a given position to another. When the operation pin 105 that is driven by the stepping motor moves the sectors 101 and 107 to the fully closed position from the small aperture position, or when the operation pin 105 moves the sectors 101 and 107 to the fully closed position from the fully open position, which causes the hunting in some cases, according to the inertial force applied to the sectors and brake on the stepping motor side. That is to say, the inertial force is applied onto the sectors 101 and 107, although they stop moving at the fully closed position from a moving state. There is a force that makes the sectors 101 and 107 overrun from the fully closed position (stop position). On the other hand, a breaking effort (braking force) is electrically or magnetically applied onto the electrically controlled stepping motor so that the rotor stops rotating in a given position. Therefore, on the fully closed position, the above-described inertial force and the braking force collide with each other to induce a vibrating state (the hunting) in moving directions of the sectors 101 and 107. If the sectors 101 and 107 vibrate in this manner, the sectors are misaligned from predetermined fully closed position and let the light come in the opening 100 (hereinafter, referred to as reexposure state). As described heretofore, it is impossible to take a photo appropriately by using the optical apparatus, such as a camera, having the shutter that cannot shield the light certainly on the originally configured fully closed position.
In Patent document 1, however, the small aperture state, fully closed state, and fully open state are formed with the use of two sectors, yet there is another type of shutter that employs a third sector for a small aperture to have three sectors in total and form the small aperture state, fully closed state, and fully open state in the same manner. The shutter having three sectors also has the above-described problem of resulting in the hunting when the sectors are moved.
In addition, it can be considered that the hunting on the above-described fully closed position can be controlled electrically. For example, there is a known technique in which while the sectors are being moved by driving the stepping motor, an electrical braking pulse is applied to control to drive the stepping motor in an opposite direction immediately before the sectors stop moving. However, as compared to the conventional shutters, the shutters these days have a high shutter speed with the brake timing width extremely narrow. Furthermore, there are differences more or less in the shutter among individual pieces after production. So, it is not realistic to adjust the brake timings of the individual pieces of mass-produced shutters. Accordingly, it's very much a situation in which it is difficult to suppress the hunting that occurs on the fully closed position by electrical control and the above-described problem of reexposure cannot be solved easily.